1. Field of the Invention
The present invention relates to thermal storage apparatuses, and more particularly to heat exchange members for use in thermal storage equipment used to store coolness and release it at a later time.
2. Description of the Prior Art
Thermal storage equipment of the type which forms ice during off peak energy periods and then uses the coolness of the ice to cool a heat transfer fluid is known, for example, from U.S. Pat. No. 4,831,831. Typically, during the ice production cycle, the heat transfer fluid is provided through a mechanical refrigeration unit to an inlet header and then passed through separate tubular passageways immersed in a pool of liquid phase change material, usually water. The heat transfer fluid is usually at a temperature below the freezing temperature of the water. This flow of subfreezing heat transfer fluid acts to form ice from the pool of water, and the ice forms annuluses encircling the individual tubular passageways. Such thermal storage equipment is typically either an internal melt or external melt type of device. In an internal melt device, during the melt cycle, after the heat transfer fluid has been warmed usually by passing through a heat exchanger in an air conditioning or cooling system, the heat transfer fluid is circulated through the individual tubular passageways to cool the fluid. The heat transfer fluid is then returned to the heat exchanger. The heat transfer fluid typically may also travel through a chiller during the melt cycle, and the thermal storage apparatus may be used to further cool the heat transfer fluid below the temperature produced by the chiller. As the heat transfer fluid is cooled in the thermal storage apparatus, the annuluses of ice surrounding the passageways melt. The cooled heat transfer fluid is re-circulated to the heat exchange apparatus, the chiller or both. In an external melt type of device, the cooled phase change material is pumped out of the tank for use outside of the thermal storage apparatus. The heat transfer fluid may vary depending on the type of equipment; for internal melt devices, ethylene or propylene glycol may be used for the heat transfer fluid; for external melt devices, ammonia or R22 may be used.
Different systems of tanks and tubular passageways have been used in such equipment. One problem with such prior thermal storage systems has been that the efficiency of the melt cycle decreases over time. As the annuluses of ice melt, annuluses of water form between the tubes and the annular jackets of ice, decreasing the efficiency of heat transfer over time, resulting in a gradual rise in the temperature of the refrigerant or heat transfer fluid leaving the thermal storage apparatus over time.
Other problems exist in the prior art thermal storage systems. In systems using tubular plastic spirals in round tanks, for example, the spirals and round tanks are commonly large, and problematic to install in a basement of an existing building. In some systems using plastic tubing, the available space in the tank is not used to its full efficiency. Systems using steel coils do not lend themselves to simple knock-down installation, that is, disassembly and re-Assembly on site, and are bulky and thus not readily moved into the basements of existing structures. Steel ice thermal storage apparatuses are also typically heavy and cumbersome to move.